1. Technical Field
The present invention relates, generally, to techniques for removing photoresist and polymeric residue from a surface of a microelectronic structure following metal etching and, more particularly, to a dry strip technique for mitigating metal corrosion during the photoresist and polymeric residue removal.
2. Background Art and Technical Problems
Microelectronic devices are often manufactured as integrated circuits (also referred to as semiconductor interconnect devices) because the various "integrated" layers of the metal stack must be interconnected to one another to facilitate the conduction of electronic signals among the various layers in the device. As device density (i.e., the number of microelectronic structures per square area) increases, the size of the various features, often referred to as the line width or critical dimension (CD), must decrease to allow an increased pattern density on a given chip area. This pressure to conserve real estate on the chip has caused many unforeseen problems in the design and manufacture of high density, deep sub-micron interconnect structures.
In particular, presently known fabrication techniques typically involve applying a layer of a photoresist material on top of a metal stack, and then exposing predetermined regions of the photoresist layer by passing a predetermined wavelength or wavelengths of light through a mask. After the desired regions of the photoresist have been exposed, the photoresist layer is developed, leaving a predetermined pattern of photoresist structures on the metal surface. The process of exposing and developing the photoresist to create this predetermined pattern is often referred to as "printing" a photoresist pattern on the metal layer.
Having established the photoresist pattern, it is then necessary to transfer this pattern into the metal, such that the photoresist pattern is suitably identically replicated in the metal layer during a metal etching process; in other words, the metal is etched away over the entire surface of the metal layer except in those regions where the photoresist pattern remains. Consequently, a well-defined pattern of metallic microelectronic structures is created in the metal layer, wherein this metal pattern very closely approximates the aforementioned photoresist pattern.
Once the pattern of metallic microelectronic structures has been created, a residual amount of photoresist often remains on the top of at least some of the microelectronic structures. In addition, some of this photoresist material often reacts with some of the gasses used in the metal etching process. It has been observed that at least some of the reacted photoresist material gets sputtered or removed from its original position and redeposited onto the side walls of the metal structures. In addition, during the metal etch process, the photoresist reacts with etch reactants and products to form "polymeric residue." Thus, after the completion of the metal etching step, many of the metal structures often exhibit polymeric residues on the side walls and some amount of photoresist remaining on top of the metallic structures.
Presently known fabrication techniques typically employ a two step process to remove the photoresist and the polymeric residue from the metal structures. More particularly, a plasma (e.g., oxygen plasma) step is typically employed to remove the photoresist. A second step, namely the removal of the polymeric residue, is typically accomplished in what is known as a wet stripping or wet strip process.
During the oxygen plasma step, the photoresist reacts with the oxygen (which is energized by the plasma) to form volatile compounds. However, some of the photoresist may remain on the substrate surface and become "hardened" as the photoresist reacts with the gases present in the plasma and other material present on the wafer surface. The hardened layer is then typically stripped from the substrate surface in a solvent solution, for example ACT 935, EKC 265, or other traditional wet strip solutions having a pH in the range of about 3 to 12. This wet strip process generally works very well in removing the polymeric residue and hardened photoresist, and is typically not problematic in the context of feature sizes on the order of one-half micron and greater. However, wet strip processes may, in addition to removing the polymeric residue and hardened photoresist, dissolve some of the metal features. If the features are large enough, this dissolution may not be problematic, but as the size of the metal features decreases, the metal dissolution may become increasingly troublesome.
Device structures employing feature sizes on the order of one-half micron and greater typically have extensions (also known as borders, dog bones, and landing pads) overlapping at least three sides of the underlying metal feature. For example, the metallic structure may extend over an underlying plug, (e.g., a tungsten plug). The extent to which a portion of the metal structure extends past the borders of the plug may vary from manufacturer to manufacturer, and may be driven by a number of design and process considerations. In effect, if the metal structure extends over the entire plug, the metal effectively isolates the plug from the wet strip medium. However, in feature sizes in the deep sub-half-micron region feature density and other design considerations often preclude the use of dog bone metal structures, essentially dictating that the metal layer overlie the underlying plug in borderless or unlanded (i.e., minimal or no extensions) relationship.
Due to various factors in the fabrication of device structures which are inherently difficult to control, such as the precise structural configuration of the metal structures, which result from the metal etch process, shrinkage and/or expansion of one or more of the metal structures and plugs, misalignment due to imperfections in the photo lithographic processes (e.g., deviations from perfect planarity), and the like, the metal structures may not always completely cover the plugs. As a result, some portion of a plug may be exposed to the wet strip solution, resulting in corrosion of the plug and/or metal structure, which corrosion may be electrochemically enhanced if the plug and overlying metallic structure are formed of different metals.
More particularly, if the electrochemical potential of the metallic structures is different from the electrochemical potential of their associated plugs, a corrosive environment may exist which can be modeled as a galvanic cell. Specifically, the metal structure and its underlying plug each function as an electrode, with the voltage bias between the two metals being a function of their respective (but different) electrode potentials. As a result of this electrovoltaic phenomenon, it has been observed that dissolution of the plugs or metal features may increase in the presence of the solvent, resulting in decreased volume of the plug and/or metal feature and, consequently, increased resistivity of the affected plugs and/or feature. The effect of this corrosion phenomenon on manufacturing throughput can be dramatic, resulting in upwards of 20% rejection rate for quarter micron device structures employing a wet strip process during manufacture. Moreover, even if these devices successfully complete resistivity and conductivity testing after manufacture, the devices may not function as desired or be reliable when subsequently used for their intended purpose.
Methods and apparatus are thus needed which facilitate the manufacture of deep sub-half-micron feature size devices, yet which mitigate the corrosive effects of presently known wet strip paradigms.